Proc. 15th INA Conf., Bohol, Philippines
J. Nannoplankton Res. 36 (2), 2016, pp.97–119 © 2016 International Nannoplankton Association
ISSN 1210-8049 Printed by The Sheridan Press, USA

97

Baltic Sea coccolithophores – an overview of insights into
their taxonomy and ecology from the last 40 years
Helge Abildhauge Thomsen
Technical University of Denmark, National Institute of Aquatic Resources (DTU Aqua), Charlottenlund Castle, Jægersborg allé 1,
DK-2920 Charlottenlund, Denmark; hat@aqua.dtu.dk
Manuscript received 28th October, 2015; revised manuscript accepted 25th January, 2016

Abstract It is an established fact that coccolithophores are of little importance with respect to biomass anddiversity in the Baltic proper. The likely biogeochemical and environmental reasons for this have recently been
critically analyzed and reviewed. The main conclusion is that the calcium carbonate saturation of the Baltic Sea is themain controlling feature, and that in particular an undersaturation during wintertime remains the critical bottleneckfor coccolithophores to prevail in the Baltic proper. While there is no reason to question these observations, it is still
relevant to put on record the actual findings of coccolithophores from the Baltic proper. Examinations of Baltic
Sea material from the Bothnian Sea, the Bothnian Bay and the Gulf of Finland prepared for transmission electron
microscopy has thus revealed a consistent presence of a low diversity community of lightly calcified coccolithophores(i.e. Balaniger virgulosa HOL and HET, Papposphaera arctica HOL cfr. and Papposphaera iugifera). When including
here also material examined from the Danish transitional waters connecting the North Sea and the Baltic proper, it
is possible to generally support the presence in the western Baltic, the Sounds and the Kattegat of a contingent ofcoccolithophores that appear to be either persistently present within the area or episodically occurring as determinedby larger scale hydrographical events within the North Sea/Baltic Sea confluence area.

Keywords coccolithophores (coccolithophorids), Baltic Sea, Kattegat

1. Introduction
It is a well-established fact the coccolithophore diversity
is very much reduced when moving from the full saline
North Sea-Skagerrak area towards the innermost brackish
parts of the Baltic Sea (Pankow, 1990; Hällfors, 2004).
Tyrell et al. (2008) have critically examined the possible
explanations behind this scenario (including salinity andsilicate concentrations) but concluded that a winter time
CaCO3 undersaturation was the most likely explanation.
The Baltic Sea winter time surface waters are, according
to their findings, distinctly undersaturated with respect toaragonite and borderline undersaturated with respect tocalcite. This leads to a situation where coccolithophores
will be unable to prevent the dissolution of their coccoliths, which in turn, in all likelihood, impacts the ecological competitiveness of these organisms.
Meier et al. (2014) provided an overview of the calcareous nannoplankton from the Kieler Förde and prepareda table that summarized the findings of coccolithophores
from the entire Baltic Sea. The current paper summarizes
observations from the last four decades of coccolithophore
research from the Baltic Sea entrance area (i.e. the Kattegat, the Sounds and the western Baltic), as well as from theinnermost and low saline parts of the Baltic Sea, with salinities ranging from 2–8. The observations presented here
are for the purpose of future studies within the region thoroughly illustrated. The description of each taxon observed
is kept as short as possible and primarily highlights newobservations that add to previous descriptions. An updated

version of the table compiled by Meier et al. (2014) is a
key element of this publication.
2. Material and Methods
Material from Danish waters (Figure 1A) comes from avariety of individual projects carried out during the last
four decades and will be summarized below in chronological order. All salinities are reported according to thepractical salinity scale 1978 (Unesco, 1981).
The ‘Belt Project’ (1975–76) had an extensive sampling
grid throughout the inner shallow Danish waters (Figure 1A).
The platform used was the RV ‘Martin Knudsen’, and theactual sampling was based on water bottles mounted in sequence on the wire on top of a CTD. Material for TEM(Transmission Electron Microscope) examination was prepared at selected stations on four occasions (October 1975;
January, March and June 1976). The station coverage oneach of these cruises (with the exception of the January
1976 cruise) is indicated in Figure 1A using color-codedsquares. Whereas the October and March cruises had fairly
complete station coverage, the June cruise did not cover
the Kattegat area because of a severe storm event. Also, theJanuary 1976 sampling was impacted by a severe storm
which resulted in a very much reduced sampling programme limited to a few stations in the Sound (not indicated in Figure 1A). The vast majority of the samples
processed for TEM are from surface waters (Figure 1B;
salinities 8–25 and 0–15°C). Non-surface samples standout clearly in Figure 1B with salinities above 25.

98ThomsenA

B

HAV90 1988-89
The Sound Sep. 2014
Isefj. #H 1984-85
Isefj. 1984-85

Skagerrak

Isefj. Tempelkrogen
Belt Project 1979-80
1975-76

North Sea
KattegatC

Oresund (the Sound)Western Baltic Sea

Belt Project (1975-76)
Seasonal coverage
Oct. 1975
March 1976
June 1976

D

1 Oct.

Kieler Förde
Meier et al. 2014

1 Jan.

E

1 Apr.

1 Jan.

1 Apr.

1 July1 Oct.

1 July(1984-85)

Figure 1: Danish sampling and basic environmental variability. (A) Map of the transition area (the Kattegat, the Belt Sea, and the western Baltic)
between the Baltic Sea proper and the North Sea. The main geographic sources of material used for this survey are identified (viz. legend in the upper
right corner). The colored squares (viz. legend in the lower left corner) show the extent of sampling during three Belt Project cruises. Also indicated(square box) is the Kieler Förde area examined by Meier et al. (2014). (B) Temperature-salinity plots of samples examined from four Belt Projectcruises as an indication of the overall annual variability in basic environmental parameters. (C) Depth profiles of temperature and salinity at a stationin the Sound sampled 25 September 2014. (D) Contour graphs (temperature and salinity) showing the annual parameter variability in the Isefjord
inner broad. (E) Contour graphs (temperature and salinity) showing the annual parameter variability at the HAV90 permanent station in the southern
Kattegat region

Baltic Sea coccolithophores....from the last 40 years

Sampling from the Isefjord has been a recurrent event
over the last four decades. The main sources of material
utilized here originate from annual sampling campaigns
conducted in the innermost broad of the fjord system(Tempelkrogen/1979–80) as well as from #H (VellerupVig) and selected stations in the Isefjord outer broad sampled during 1984–85. Sampling was either from the shore(Tempelkrogen) or utilizing small boats, i.e. a Universityof Copenhagen dinghy at #H or a torpedo recovery vessel from the Kongsøre Naval Facility in the Isefjord outerbroad. The Isefjord system maintains a fairly high salinity(>18) due to a frequent exchange of water through the entrance to the Kattegat and also because of a rather limited
drainage area. The temperature varies annually between 0
and 20°C. The Isefjord system is very shallow and characterized by a water column that rarely becomes stratified(Figure 1D).
A large scale and multifaceted research programme(‘HAV 90’) was conducted using the University of
Copenhagen RV ‘Ophelia’ at a station in the southern Kattegat (Figure 1A) during 1988–89. The water
depth exceeds 20m and the water column is distinctly
temperature and salinity stratified during the summer
months (Figure 1E). Samples intended for TEM analysis originate from surface waters and from the fluorescence maximum depth, which is typically coincident
with the halocline.
Material collected on an ad hoc basis from various
sites within the western Baltic/Kattegat region has also
been considered with reference to this paper. Details ona specific sample will be added where relevant. More recently, single event sampling has been conducted in, e.g.
the Sound (Figure 1A/Sep. 2014). Sampling was carriedout from the University of Copenhagen RV ‘Ophelia’.
The samples analyzed originate from the brackish surface
layer (5m) and from the full saline deeper waters (20m).
Vertical temperature and salinity profiles are shown in
Figure 1C.
Sampling from the innermost parts of the Baltic
Sea took place on several occasions (Figure 2A). The
Tvärminne Zoological Station was thus visited twice(1974, 1978) and extensively sampled using boats andsampling equipment from the station at sites covering theopen sea, the archipelago and the near costal fjord systems(see e.g. Thomsen, 1979). A large scale coverage of the innermost parts of the Baltic Sea was made possible duringcruises on the old RV ‘Aranda’ during July–August 1979
(Figure 2A) and on the new RV ‘Aranda’ during March
1994 (Figure 2A) as part of the OSIC 1994 winter expedition (see e.g. Ikävalko & Thomsen, 1997). Temperatureand salinity profiles (July–August 1979) from the Gulf ofFinland transect, as well as a station sequence covering
the Northern Baltic, Bothnian Sea and Bothnian Bay stations, are shown in Figure 2F, G. Surface salinity is around99

6 in the Gulf of Finland and decreasing towards 3 at thenorthernmost Bothnian Bay station. Most of the samples
processed for TEM during the 1979 cruise are from theuppermost 5m of the water column. Deeper water samples(25 and 50m) were processed only at selected stations. The
sampling effort during the OSIC 1994 expedition focusedon the ice biota and included only a few water column
samples.
The protocol for processing water samples for the
TEM was similar on all sampling occasions (see Moestrup& Thomsen, 1980). The nannoplankton community wasconcentrated for further processing by means of either
centrifugation of a prefiltered (usually 20µm) water sample (0.5–1.0l) or centrifugation of prefiltered material resuspended from an initial filtration of cells on top of, e.g.
a 1µm Nuclepore filter. Small droplets of cells from theresuspended final pellet of material were placed on carbon
coated grids for the TEM. Cells were subsequently fixed
for c. 30 seconds in the vapor from a 1–2% solution of
OsO4. After drying the grids were carefully rinsed in distilled water in order to remove salt crystals. Grids wereshadow cast with either Au/Pd or Cr prior to the examination in JEOL electron microscopes property of the Botanical Institute at the University of Copenhagen (JEM-T8,
JEOL 100SX).
Material for the SEM (Scanning Electron Microscope) was in some instances (e.g. Pl. 2, fig. 2) standard
Au-coated TEM grids prepared according to the techniqueoutlined above and examined in a JEOL 100CX fitted witha high resolution scanning device (joint property of the
Botanical and Genetics Inst, Univ. Copenhagen). Imagesof cells from the Sound (Pl. 1, figs 1, 7, 11; Pl. 2, fig. 7)
originate from Au-coated coverslip preparations that wereprepared in accordance with the TEM grid routine andexamined in a FEI Quanta 200 ESEM FEG (property of
DTU Cen). The Bergen SEM material (Pl. 6, fig. 3) was
prepared by gentle filtration of a water sample on top of
a 1.0µm Nuclepore filter. The formation of salt crystals
that might obstruct the visibility of cells was minimizedby allowing the pumping system to almost completely
dry out the filter. Filters were sputter coated with goldand examined on a Zeiss Supra 55VP scanning electron
microscope at the Bergen University Laboratory for Electron Microscopy.
The taxonomic-morphological terminology used here
largely follows Young et al. (1997, 2003 and references
therein). There is no exact quantitative data available tosupport the qualitative information assembled here. Whenan organism is described as frequent or abundant, it ismerely a subjective statement based on the number of cells
encountered when examining the non-quantitative TEM
preparations.
The exact origin of material selected for illustration
appears in Table 1.

100ThomsenA

Bothnian Bay

B

Balaniger
virgulosa HOL

25, 50 m

50 m

RR6
BO3

C

Balaniger
virgulosa HET

25 m
25 m

F16

25, 50 m

0, 25, 50 m

F18
F18

0, 20, 45 m

F26
Surface samples when
not otherwise specified

Bothnian
SR5
Sea

Tvärminne Zool. St.
RV Aranda 1979
RV Aranda 1994

LL3
F71

Gulf of
Finland

LL15

D

Papposphaera
arctica HOL cfr.

25 m

E

Papposphaera
iugifera HET

25 m

25, 50 m

25, 50 m

Baltic Sea
Tvärminne Zool. Station

1974, 1978

Bornholm

RV Aranda Mar. 1994
RV Aranda July-Aug. 1979

Surface samples when
not otherwise specified

Tvärminne Zool. St.
RV Aranda 1979
RV Aranda 1994

F

G

Northern Baltic

Bothnian Sea

Bothnian Bay

West North. Baltic

Gulf of Finaland

East

Figure 2: Sampling from the Gulf of Finland, Bothnian Sea and Bothnian Bay, occurrence patterns of selected species, and basic environmental vari­ability. (A) Map of sampling sites distributed among two RV ‘Aranda’ cruises and multiple visits to the Tvärminne Zoological Station and its immediatesurroundings. (B) Balaniger virgulosa HOL. (C) Balaniger virgulosa HET. (D) Papposphaera arctica HOL cfr. (E) Papposphaera iugifera HET.
(F) Contour graphs (temperature and salinity) showing the variability along a south to north transect in the Bothnian Sea and the Bothnian Bay.
(G) Contour graphs (temperature and salinity) along a west to east transect in the Gulf of Finland

Baltic Sea coccolithophores....from the last 40 years

101

Plate

Taxon

Illustration

Origin

Date

Misc.

1

Syracosphaera nodosaFig. 1

Denmark, the Sound

Sept. 2014

20m depth

Calciopappus caudatus

Figs 2–4

Denmark, Helgenæs, Kattegat

June 1976

15m depth

Ceratolithus cristatus

Figs 5–6

Denmark, Hals, N. Kattegat

Oct. 1975

Surface sample

Algirosphaera robusta

Fig. 7

Denmark, the Sound

Sept. 2014

20m depth

Syracosphaera corolla

Fig. 8Denmark, Tempelkrogen, the Isefjord

Feb. 2015

Surface sample

Braarudosphaera bigelowii

Figs 9–10

Denmark, Tempelkrogen, the Isefjord

Sept. 1980

Surface sample

Emiliania huxleyi

Fig. 11

Denmark, the Sound

Sept. 2014

20m depth

Pleurochrysis roscoffensis cfr.

Figs 12–13

Finland, Ajax, Tvärminne Zool. Station

Aug. 1978

Surface sample

Acanthoica quattrospina HOL

Figs 1–6

Denmark, the Isefjord outer broad

Aug. 1984

Surface sample

Acanthoica quattrospina HET

Fig. 7

Denmark, the Sound

Sept. 2014

20m depth

Fig. 8Denmark, the Isefjord outer broad

Aug. 1984

Surface sample

Syracosphaera borealis

Figs 1–4

Denmark, the Isefjord outer broad

Aug. 1984

Surface sample

Corisphaera strigilis cfr.

Fig. 5

Denmark, HAV90 station, S. Kattegat

Sept. 1989

Surface sample

4

Corisphaera strigilis cfr.

Figs 1–5

Denmark, the Isefjord outer broad

Aug. 1984

Surface sample

5

Balaniger virgulosa HOL

Fig. 1

Denmark, Køge Bight, W. Baltic

Oct. 1977

Surface sample

Fig. 2

Denmark, the Isefjord outer broad

Aug. 1984

Surface sample

Figs 3–5

Denmark, Tempelkrogen, the Isefjord

Sept. 1980

Surface sample

Balaniger virgulosa HET and HOL

Figs 1–2

Finland, Bothnian Sea, #SR5

Aug. 1979

25m depth

Balaniger virgulosa HET

Fig. 3

Norway, Bergen area

Fig. 4

Denmark, coastal North Sea

July 1975

Surface sample

Fig. 5

Denmark, N. Kattegat

March 1976

15m depth

Fig. 1Finland, Bothnian Sea, #SR5

Aug. 1979

25m depth

Fig. 2Finland, Bothnian Sea, #SR5

Aug. 1979

50m depth

Fig. 3Finland, Bothnian Sea, #SR5

Aug. 1979

25m depth

Fig. 4

Finland, Bothnian Bay, #BO3

Aug. 1979

50m depth

Fig. 5Finland, Bothnian Sea, #SR5

Aug. 1979

50m depth

Fig. 6

Finland, Bothnian Sea, #F16

Aug. 1979

25m depth

Fig. 1

Denmark, Læsø Ø, N. Kattegat

Mar. 1976

15m depth

Fig. 2

Denmark, Als, W. Baltic

June 1976

Surface sample

Fig. 3

Denmark, Læsø Ø, N. Kattegat

Mar. 1976

15m depth

Fig. 4

Denmark, Kolby Kås, S. Kattegat

June 1976

10m depth

Figs 5–6

Denmark, Læsø Ø, N. Kattegat

Mar. 1976

15m depth

Fig. 7

Denmark, Endelave, S. Kattegat

June 1976

Surface sample

Fig. 8Denmark, Als, W. Baltic

Oct. 1975

Surface sample

Fig. 9Denmark, Als, W. Baltic

June 1976

Surface sample

Fig. 10

Denmark, HAV90 station, S. Kattegat

April 1989

Surface sample

Fig. 1

Denmark, Knudshoved, the Great Belt

Mar. 1976

Surface sample

Figs 2–3

Denmark, Drogden, the Sound

Mar. 1976

Surface sample

Figs 4–5

Denmark, Stevns, W. Baltic

Jan. 1976

Surface sample

Figs 6, 8

Finland, Bothnian Sea, #SR5

Aug. 1979

25m depth

Fig. 7

Finland, Bothnian Sea, #F71

Aug. 1979

45m depth

Papposphaera lepida

Figs 1–2

Denmark, the Sound

Jan. 1976

10m depth

Pseudowigwamma scenozonion

Figs 3–4

Denmark, Læsø Rende, N. Kattegat

Mar. 1976

10m depth

Wigwamma arctica

Fig. 5

Denmark, Læsø E, N. Kattegat

Mar. 1976

15m depth

2

3

6

7

8

Balaniger virgulosa HET

Trigonaspis minutissima

Pappomonas borealis HOL
Pappomonas borealis HET

9Papposphaera arctica HOL

Papposphaera arctica HOL cfr.
10

Table 1: Overview of the origin of material selected for illustration (Plate 1–10)

3. Results and discussion
3.1 Isochrysidales
3.1.1 Noelaerhabdaceae
Emiliania huxleyi (Lohmann, 1902) Hay & Mohler in
Hay et al., 1967
Pl. 1, fig. 11
The only variety observed in inner Danish waters is
E. huxleyi type A. This taxon occurs year round in inner
Danish waters (Figure 3A) and sometimes forms dense
blooms in the northern Kattegat region (Andersson &­Eilola, 2006). The recorded temperature and salinity intervals are 0.3–14.5°C and 8.4–31.7, respectively. Emiliania
huxleyi has been observed as far east as Bornholm (May1975/see Figure 2A).

3.1.2 Pleurochrysidaceae
Pleurochrysis roscoffensis (Dangeard, 1934) Fresnel &Billard, 1991 cfr.
Pl. 1, figs 12–13
Observed twice in near-coastal samples from sites nearby the
Tvärminne Zoological Station. It appears from Pl. 1, fig. 13that this species has nodes on both the distal shield and thetube inner wall which are characteristics of P. roscoffensis.
3.2 Syracosphaerales
3.2.1 Rhabdosphaeraceae
Acanthoica quattrospina Lohmann,
1903/Sphaerocalyptra sp.
Pl. 2, figs 1–8

102ThomsenA

Emiliania
huxleyiB

Calciopappus
caudatus

C

Balaniger
virgulosa HOL

Belt Project

Oct. 1975
March 1976
June 1976

D

Balaniger
virgulosa HET

E

Trigonaspis
minutissima

F

Pappomonas
flabellifera

North Sea coastal
sampling site (June 1975)

G

Papposphaera
arctica HOL

20 Jan. 1976

Figure 3: Findings of selected coccolithophores from inner Danish waters. (A) Emiliania huxleyi; (B) Calciopappus caudatus; (C) Balaniger virgu­
losa HOL; (D) Balaniger virgulosa HET; (E) Trigonaspis minutissima; (F) Pappomonas flabellifera HET; (G) Papposphaera arctica HOL

A life cycle involving Acanthoica quattrospina (Pl. 2, figs 7–8)
and a previously undescribed holococcolithophore with affinity to Sphaerocalyptra sp. has been suggested on multiple occasions (e.g. Cros et al., 2000; Cros & Fortuño, 2002;
Meier et al., 2014; Šupraha et al., 2014). Combination

coccospheres are missing in our material. However, the twostages co-occurred and were abundantly present in samples
from the Isefjord Outer Broad during August 1985.
Complete cells of A. quattrospina HOL are shown in
Pl. 2, figs 1–3. The coccosphere has a triangular outline

Baltic Sea coccolithophores....from the last 40 years(width × length c. 4 × 5µm) and carries two flagella(c. 14µm) and a markedly shorter haptonema (c. 7µm).
Little information can be obtained from Pl. 2, fig. 1 onfeatures of the individual holococcoliths due to the in­ability of the electrons to pass through the coccosphere.
However, it is at least apparent that there is a marked
difference between circumflagellar and body coccoliths
in terms of the degree to which the central part of thecoccolith protrudes above the base plate. It also appears
that the most completely developed calyptroliths havea proximal part with parallel sides from where the cone
eventually arises. Also, the cell shown in Pl. 2, fig. 3 has a
haptonema. This is curled up and appears to be somewhat
longer than the one shown in Pl. 2, fig. 1. The TEM examination of A. quattrospina HOL (=Sphaerocalyptra sp.)
provides new information on, in particular, the unmineralized parts of the Sphaerocalyptra periplast. Below the large(1.6 × 1.25µm), oval organic scales that support the calyptroliths (Pl. 2, fig. 3) there are two additional layers of organic
scales, i.e. medium-sized oval plates (1.0 × 0.8µm) and small
scales (0.25 × 0.2µm) that tend to occur attached to the periphery of the latter (Pl. 2, figs 4, 6). The medium-sized scales
in particular clearly show the characteristic radiating surface
patterning typical of a haptophyte scale. In some instances,
the surface of these is covered by organic material in the
shape of ‘columnar’ structures (Pl. 2, fig. 5). Similar material has previously been observed in, e.g. Chrysochromulina
herdlensis (Leadbeater, 1972; Manton & Leadbeater, 1974).
Algirosphaera robusta (Lohmann, 1902) Norris, 1984
Pl. 1, fig. 7This species was fairly frequent in the September 14
sample from the Sound (20m; see Figure 1C). Meieret al. (2014) first reported this species from the Baltic Sea(Kieler Förde) and concluded that it was not likely a native
species but rather a temporary intruder from the North Sea
where it is a well-established taxon (Charalampopoulou
et al., 2011). Its co-occurrence in the 20m sample from theSound, together with, e.g. a group of non-native loricate
choanoflagellates (Thomsen, unpublished observations),
points to that its occurrence in inner Danish waters andthe western part of the Baltic Sea is indeed linked to special hydrographical events within the area. Algirosphaera
robusta has been reported to form combination coccospheres with Sphaerocalyptra quadridentata (Schiller,
1913) Deflandre, 1952 (see e.g. Triantaphyllou & Dimiza,2003). This life-phase has not so far been reported fromthe Baltic Sea.
3.2.2 Syracosphaeraceae
Calciopappus caudatus Gaarder & Ramsfjell, 1954
Pl. 1, figs 2–4
Commonly observed June 1975 and also found in September 2014 (Figure 3B). Flagellated cells with a haptonema(Pl. 1, fig. 3) and small under layer scales (Pl. 1, fig. 4) aresometimes observed.

103

Syracosphaera borealis Okada & McIntyre, 1977
Pl. 3, figs 1–4
Syracosphaera borealis was also observed by Meier et al.
(2014) from the Kieler Förde. The axial structure, which istypically developed as a prominent irregular ridge formed
by upgrowth from laths, is little pronounced in the Isefjord
material (Pl. 3, figs 1–3). However, when examining the
Kieler Förde material presented by Meier et al. (2014; theirPl. 3, figs 1–5) it is evident that this feature is highly variable
across coccoliths from the very same coccosphere (i.e. fromabsent to pronounced), so there is every evidence to believe –considering also the proximity of the geographic sites - that
the Isefjord 1975 material is also correctly identified as
S. borealis. The coccoliths illustrated in Pl. 3, fig. 4 are furnished with irregular and isolated nodes which is a featureassigned to S. borealis type 2 by Kleijne (1993).
Syracosphaera corolla Lecal, 1966
Pl. 1, fig. 8
A single S. corolla exothecal coccolith (Pl. 1, fig. 8) was
observed recently (February 2015) from the Isefjord area.
Considering that S. corolla is ubiquitously present inNorth Sea waters (e.g. Charalampopoulou et al., 2011) it
is likely that this taxon will occasionally manifest itself
also inside the North Sea-Baltic Sea boundary.
Syracosphaera nodosa Kamptner, 1941 cfr.
Pl. 1, fig. 1
Commonly observed in the September 2014 sample fromthe Sound (20m depth). Not previously recorded fromthe Baltic Sea. Syracosphaera nodosa is reported to form
combination coccospheres with Helladosphaera cornifera(Schiller, 1913) Kamptner, 1937, a life-phase which has
not so far been observed in the Baltic Sea.
3.3 Holococcolithophores
Corisphaera strigilis Gaarder, 1962 cfr.
Pl. 3, fig. 5; Pl. 4, figs 1–5
Frequently observed during the August 1985 coccolithophore ‘bloom’ in the Isefjord (Pl. 4, figs 1–5). Also
observed in a single sample from the HAV90 southern
Kattegat station (Pl. 3, fig. 5).
The conical cell measures c. 5µm and carries two flagella and a somewhat shorter haptonema (Pl. 4, figs 1, 4).
The periplast comprises two layers of organic scalesof which the largest and outermost scales (1–1.3 ×
0.7–0.9µm) are mineralized. There is an underlayer ofsmall scales (0.4–0.45 × 0.3–0.35µm) that are likely inthe undisturbed periplast located along the periphery of
the larger mineralized scales. The scatter of scales (Pl. 3,
fig. 5) seems to indicate that there is also a medium
sized, non-mineralized scale type (left side of the image,arrows). However, it appears more likely, considering the obvious calcification gradient across the image,that these are probably completely decalcified ‘coccoliths’. The C. strigilis body coccoliths are formed by104Thomsenangular crystallites arranged in regular patterns. Fromthe micrographs available it appears that the coccoliths
have no peripheral tube (as in, e.g. the closely related
C. gracilis Kamptner, 1937). The calcification is rather
limited to a peripheral band (c. 3 crystallite lines wide)
which is spanned by a bridge (Pl. 4, fig. 5; single coccoliths from Pl. 3, fig. 5 and Pl. 4, fig. 4). Coccoliths in
C. strigilis are dimorphic with flagellar pole coccoliths
carrying a bridge that is extended into a leaf-like lamina.
This is not evident in our material but at least hinted at in
Pl. 4, fig. 4 where some anterior end coccoliths have arched
median structures.
The facts that C. strigilis has been found (Šupraha
et al., 2015) to share a combination coccosphere with
Syracosphaera hirsuta Kleijne & Cros, 2009 (formerly
S. borealis type 2), and also that a very distinct co-occurrenceof S. borealis and C. strigilis is reported from the Southern
Ocean (Malinverno, pers. com.), adds further evidence to theidentification of the material described here as representing
form complexes of C. strigilis and S. borealis, respectively.
3.4 Coccolith families incertae sedis
3.4.1 Papposphaeraceae
Balaniger virgulosa (Thomsen & Oates, 1978) Thomsen& Østergaard, 2014
The established terminology used to distinguish life cycle phases (HET and HOL) is used also in the context of
B. virgulosa although in this species the haploid phaseproduces calcite elements (nannoliths) which are morphologically significantly different from holococcolith crystallites in the strict sense. An update of terminology will
likely be needed at some stage to address this issue.
Balaniger virgulosa HOL
Pl. 5, figs 1–5; Pl. 6, figs 1–2
Syn.: Balaniger balticus Thomsen & Oates, 1978
Balaniger virgulosa HOL is, despite its small overall dimensions (L = 3–5µm; W = 4–6µm), easily recognizedeven at the light microscope level due to its distinct heartshaped cell contours, the two flagella and a haptonema,
and the apoplastidic nature of the cell. When examined in
the TEM the species is unequivocally identified from thepresence of minute hollow pyramids (Pl. 5, figs 1–3) that
constitute the coccosphere in this species. The presence of
calcium in the periplast was verified when the species was
first described (Thomsen & Oates, 1978). Decalcification
experiments showed (Thomsen & Oates, 1978) that thepyramid is formed of organic material that does not change
in overall appearance following acid treatment. The angular appearance of some pyramid distal tips (e.g. Pl. 5, fig. 1,
arrows) points to that this is where a crystallite might belocated. Groups of pyramids (or ‘hats’ as they are also
­often referred to) are attached to organic plates that cover
the entire surface of the cell (Pl. 8, figs 4–5). Thomsen &Østergaard (2014) documented that B. balticus shares alife cycle with Pappomonas virgulosa, hence the recent

name change. Material of B. virgulosa HOL from theinnermost parts of the Baltic Sea is included here as Pl. 6,
figs 1–2. The shape of the individual ‘hat’ in material fromthe Baltic Sea proper is mostly of the sharp-pointed type
(Pl. 6, fig. 2) that has also infrequently been observed in material from inner Danish waters (Thomsen & Oates, 1978/l.c.
Pl. 3 B, C). It is tempting to hypothesize that the differencein ‘hat’ design is perhaps due to a complete lack of calcification in the genuine Baltic Sea material of this taxon.
The type locality of B. virgulosa HOL (i.e. B. balticus)
is Tempelkrogen, the innermost part of the Isefjord system. Since its first description this coccolithophore species
has been repeatedly observed in the Isefjord throughout
the years and seasons sampled. Findings outside the fjord
system are, however, rather sparse and scattered (Figure 3C).
Balaniger virgulosa HOL is distributed throughout theentire Baltic Sea. It has been observed repeatedly in samples collected in the vicinity of the Tvärminne Zoological
Station (Thomsen, 1979; Vørs, 1992) and was also foundat several stations occupied by RV ‘Aranda’ cruises in1979 and 1994 (Figure 2B). Balaniger virgulosa HOL isboth eurythermal and euryhaline.
Balaniger virgulosa HET
Pl. 6, figs 1–5; Pl.7, figs 1–6
Bas.: Pappomonas virgulosa Manton & Sutherland, 1975
Recently Thomsen & Østergaard (2014) reexamined Ba­
laniger virgulosa HET based on material from the Arctic. The
typical Atlantic Ocean appearance of this life history phase(Pl. 6, fig. 5) is here further documented from a SEM image(Pl. 6, fig. 3; courtesy of M. Heldal, Univ. of Bergen) of material from the Bergen area, Norway. Balaniger virgulosa HET
carries oval body coccoliths and circumflagellar coccoliths
that terminate in a tuft of elongated elements (Pl. 6, fig. 3).
The rim and central area calcification comprises linearly arranged sequences of erect, elongated elements separated bysmall adnate, rod-shaped elements (Pl. 6, fig. 4). Coccoliths
at the antapical pole consistently carry reduced central appendages (shaft reduced or sometimes even missing). It appears from Pl. 6, fig. 3 that ordinary body coccoliths may also
occasionally be furnished with such reduced appendages.
The Danish material (viz. Pl. 6, figs 4–5) matches theappearance of this taxon from high latitude sites fairly well(Manton & Sutherland, 1975; Thomsen & Østergaard,2014). When examining material from the innermost partsof the Baltic Sea (Pl. 7) it becomes evident that there are twomorphotypes of this species present within the area. The cellillustrated in Pl. 7, figs 4, 6 is characterized by a very clear
distinction between flagellar pole coccoliths with central appendages and body coccoliths that are completely devoid of
these. Coccoliths from the antapical pole (Pl. 7, fig. 4, arrow)carry reduced central appendages where the shaft is missing.
However, in the majority of the cells examined from the Baltic Sea proper (e.g. Pl. 7, figs 1, 5) all coccoliths carry central
appendages, yet with a marked distinction in size from theapical towards the antapical pole. In this respect, these cells

Baltic Sea coccolithophores....from the last 40 years

are thus reminiscent of the Bergen material (Pl. 6, fig. 3).
While the cells illustrated in Pl. 7 are selected to represent
the extreme ends of the observed variability, it should beemphasized that intermediate forms do also occur. It appears
acceptable, based on the patterns of variability observed
across the entire geographic range of B. virgulosa, andwithout access to information beyond what can be retrieved
from chance observations of natural samples, to argue that
this variability is likely reflecting environmental impact andmust, therefore, be accounted for within the concept of a single species. Teratological modifications of the B. virgulosa
calyx (solid, flattened V-like structures) sometimes occur inmaterial from the Baltic Sea proper (Pl. 7, figs 2–3).
The findings of B. virgulosa HET from inner Danish
waters as well as the innermost parts of the Baltic Sea areshown in Figure 3D and Figure 2C. It is evident that thisspecies is both eurythermal and euryhaline with a potential
to occur throughout the entire Baltic Sea at any specifictime of the year. It deserves to be mentioned that whereas
B. virgulosa HOL is frequently recorded from the Isefjord
area we have never succeeded in finding B. virgulosa HET
inside the fjord, despite the fact that this life history stage
has been frequently recorded from the nearby ‘HAV90’
southern Kattegat permanent station.

105

supports numerous triangular groups of crystallites (Pl. 8,
fig. 6) organized in helical paths along the surface of a‘tower’ (see also Thomsen and Østergaard, 2015).
Findings of P. borealis HOL are restricted to the northernmost Kattegat area and only during wintertime and in
subsurface samples (17 March 1976, 3–4°C, salinity range
29.5–31.7).
Pappomonas borealis HET
Pl. 8, fig. 10
Pappomonas borealis HET has recently (Thomsen &Østergaard, 2014) been redefined as a separate taxon (formerly referred to as P. flabellifera var. borealis) based on
differences in the design of the flagellar pole coccoliths,
and also size and central area calcification of the body coccoliths.
Pappomonas borealis HET has so far only been foundtwice in Danish waters (western Baltic [Als], June 1976,
12.3°C, salinity 17.6; southern Kattegat [HAV90], April
1989, 7°C, salinity 14).
Pappomonas flabellifera Manton & Oates, 1975 emend.
Thomsen & Østergaard, 2014
Pl. 8, figs 8–9
The Danish material of P. flabellifera (Pl. 8, figs 8–9) mirrors exactly the Arctic material of this taxon as illustrated
by, e.g. Thomsen & Østergaard (2014; l.c. Fig. 3) withreference to morphological details as well as dimensions.
Pappomonas flabellifera has been found on several occasions (Figure 3F) within a temperature range of 4–12°C
and at salinities ranging from roughly 12–32. The species isadditionally known from the coastal North Sea (Figure 3F)
in samples dating back to June 1975 (15°C, salinity 32).

Trigonaspis minutissima Thomsen, 1980
Pl. 8, figs 1–4
Trigonaspis minutissima is a diminutive coccolithophore
species, 2.4–3µm in diameter, with the usual two flagella(14–21µm) and a somewhat shorter haptonema (8–12µm).
The cell carries a tuft of narrow, tower-shaped flagellar
pole coccoliths (Pl. 8, fig. 1). Calcification is in the formof triangular groups of crystallites. For further details see
Thomsen & Østergaard (2015). There is no evidence so
far of a life cycle counterpart to the holococcolithophore
Papposphaera arctica (Manton, Sutherland & Oates,
T. minutissima. However, indirect evidence (e.g. co-­1976) Thomsen, Østergaard & Hansen, 1991 HOL
occurrence) supports the idea that Pappomonas flabelli­Pl. 9, figs 1–5
fera might be a strong candidate.
Bas. Turrisphaera arctica Manton, Sutherland & Oates,
Findings of T. minutissima are summarized in Figure 3E.
1976
The overall temperature and salinity ranges are 3–12.3°C Based on the finding of combination coccospheres, Thomsen
and 17.6–31.7, respectively.
et al. (1991) demonstrated that species of Papposphaera
share a life history with species of Turrisphaera. This
Pappomonas borealis (Manton, Sutherland & McCully,
has since been further substantiated in, e.g. a series1976) Thomsen & Østergaard, 2014of publications on species of Papposphaera (Thomsen
The shared life cycle between P. borealis and T. diskoensis et al., 2015a, b; 2016). Turrisphaera spp. are characterized
was first documented by Thomsen et al. (1991) and has by a coccosphere that comprises tower-shaped coccoliths
later been confirmed by the finding of additional combi- where an organic matrix supports a surface monolayer
nation coccospheres in West Greenland waters (Thomsen of hexagonal groups of crystallites. The three speciesand Østergaard, 2014).
described so far under the genus name Turrisphaera(borealis, arctica, polybotrys) are distinguished primarily
Pappomonas borealis HOL
on the basis of morphological features with reference toPl. 8, figs 5–7
the distal part of the flagellar pole coccoliths (Manton
Syn. Trigonaspis diskoensis Thomsen, 1980
et al., 1976; Thomsen, 1980). In T. arctica the flagellar pole
Pappomonas borealis HOL is primarily characterized by a coccoliths are thus unilaterally hypertrophied towards thetuft of rather prominent, tower-shaped flagellar pole coc- distal end. This particular feature is also evident in imcoliths (Pl. 8, figs 5, 7) formed by an organic matrix that ages of Turrisphaera sp. from inner Danish waters (Pl. 9,

106Thomsenfigs 1–3). It has recently been shown (Thomsen et al., 2015a)that T. arctica shares a life history with a Papposphaera sp.
that differed from previously described species of Pappo­
sphaera. In this species the coccolith central area calcification is in the shape of a simple axial cross and the calicate
spines are of the P. sagittifera type (see e.g. Thomsen, 1981).
In accordance with these recent observations the material
examined here is, therefore, formally referred to as P. ­arctica
HOL. A single combination coccosphere was found frominner Danish waters (Pl. 9, figs 4–5). While it is obviously
T. arctica based on the asymmetrical flagellar pole coccoliths(Pl. 9, fig. 4, arrow), it is unfortunately not possible to fully
account for the identity of the Papposphaera sp. (Pl. 9, fig. 5).
While the rim calcification is clearly of the Papposphaera
type, it is not possible to analyze the pattern of central area
calcification which is critical in this context.
Papposphaera arctica HOL is fairly frequently observed in inner Danish waters (Figure 3G) at temperatures and salinities ranging from 1.6–3.6°C and 9.3–19.2,
respectively. The finding of P. arctica in a June sample(14.2°C) somehow ‘spoils’ the picture of a stenothermaland euryhaline occurrence pattern.
Papposphaera arctica (Manton, Sutherland & Oates,
1976) Thomsen, Østergaard & Hansen, 1991 HOL cfr.
Pl. 9, figs 6–8
Whereas there is no doubt about the generic affiliation ofthis material (Turrisphaera) based on the presence of hexagonal groups of crystallites (Pl. 9, fig. 8), the Bothnian Sea
material of P. arctica HOL cfr. (Pl. 9, figs 6–7) unfortunately provides little information with respect to the designof the flagellar pole coccolith, which is the key to species
identification. The material is here tentatively referred to as
P. arctica HOL cfr. despite the fact that it is not possible toconvincingly demonstrate the presence of the asymmetricalcircumflagellar coccoliths that is the key figure of this taxon
in this material (see e.g. Pl. 9, fig. 2). The logic behind illustrating the material under this heading is the fact that thisspecies tends to be the most frequently observed form ofTurrisphaera in the confluence area between the Baltic Sea
and the North Sea. It is tempting to hypothesize, considering that Papposphaera iugifera (Thomsen & Østergaard,2016) is observed from the very same Bothnian Sea stations (Figure 2E), that the specimens illustrated here could
rather be the thus far unknown life history counterpart ofthis newly described species of Papposphaera.
The occurrence of P. arctica HOL cfr. (Figure 2D) islimited to only two stations (SR5, 25m, 2 August 1979;
F71, 25 and 50m, 1 August 1979).
Papposphaera lepida Tangen, 1972
Pl. 10, figs 1–2
Papposphaera lepida is an easily recognizable species thatwas originally described from the Bergen area (western Norway) and since then observed worldwide (Tangen, 1972). The
coccosphere comprises monomorphic coccoliths furnishedwith a highly conspicuous calicate spine. The calyx is a flattened, circular structure formed from four separate elements
and placed perpendicular to the stem (Pl. 10, fig. 1). There is acertain size variability among the individual calyces depending on their position within the coccosphere – with the largest
ones clustering around the apical pole. A wristlet supports thetransition from stem to calyx (Pl. 10, fig. 2).
Papposphaera lepida has only been observed twicefrom the entire Baltic Sea region: south-western Kattegat area (16 June 1976, 15m, 6.3°C, salinity 31.4), northernmost entrance to the Sound (19 January 1976, 2.7°C,
salinity 26.3). Tangen (1972) summarized that P. lepida
occurred within a wide range of both temperatures (6.5–
20.5°C) and salinities (10.3–30.3). Considering that thefindings reported here extend the lower temperature limit
a few degrees and that the species is also known from,e.g. Thailand coastal waters (Thomsen, unpublished), it
is obvious that this is a truly eurythermic species. At thesame time there is an indication that brackish water (<10;e.g. the Baltic Sea east of Bornholm) is likely to present abarrier towards the distribution of this species.
Pseudowigwamma scenozonion (Thomsen, 1980) Thomsen
in Thomsen, Østergaard & Heldal, 2013
Pl. 10, figs 3–4
Bas.: Wigwamma scenozonion Thomsen, 1980
The coccolith calcification in this tiny (4–6µm) species islimited to the rim of an otherwise uncalcified plate (Pl. 10,
fig. 3). A characteristic feature of the species is the occurrenceof a single, enlarged and almost quadratic element (Pl. 10,
fig. 4) among the normal rod-shaped elements forming theperipheral rim. The transfer of this species to a monotypic
genus (Thomsen et al., 2013) was argued by significant differences in rim architecture when comparing this species to
Wigwamma spp., and the lack of a coccolith superstructure.
Finally, there is also circumstantial evidence (Thomsen et al.,2013) indicating that P. scenozonion has a life cycle counterpart that deviates markedly from that of Wigwamma spp.
Pseudowigwamma scenozonion occurred only once insamples from the Baltic Sea at large, i.e. at a station in thenorthernmost part of the Kattegat (Læsø west, 17 March
1976, 10m, 3.0°C, salinity 29.5).
Wigwamma arctica Manton, Sutherland & Oates, 1977
Pl. 10, fig. 5
Thomsen et al. (2013) recently reexamined the genus Wig­
wamma and all described species. While doing so they
were looking for consistent patterns in the morphological variability of, e.g. W. arctica. Features such as how
the arms of the pyramidal coccolith superstructure meet(single point versus one rod protruding distally above theothers), and also details in the assembly of coccolith rim
and superstructure (enlargement versus no enlargement of
the rim elements that ‘carry’ the superstructure), were usedin an effort to search for obvious morphological patterns
reflecting large scale regions of occurrence. The approach

Baltic Sea coccolithophores....from the last 40 years

taken was not adequate even in terms of distinguishing
between Arctic and Antarctic populations of W. arctica. It
was therefore concluded that the variability observed had
to be dealt with within the context of a single species. The
Danish specimen illustrated here (Pl. 10, fig. 5; one spine
protruding and rim crystallite enlargement) clusters with
Antarctic material rather than material from either the West
Greenland type locality or NE Greenland (NEW polynya).
Wigwamma arctica occurred only twice in samples fromthe Baltic Sea at large, i.e. at a station in the northernmost partof the Kattegat (Læsø east, 17 March 1976, surface: 0.5°C,
salinity19.1, and at 15m depth: 4.0°C, salinity 31.7).
3.5 Nannolith families incertae sedis
3.5.1 Braarudosphaeraceae
Braarudosphaera bigelowii (Gran & Braarud, 1935)
Deflandre, 1947
Pl. 1, figs 9–10
The B. bigelowii coccosphere is a regular, pentagonal dodecahedron (Pl. 1, fig. 9). The individual liths are pentagonal (Pl. 1, fig. 10) and with a side length of c. 5µm.
Observed a few times in samples from the Isefjord inner
broad. Hagino et al. (2013) showed that in the 18S rDNA
tree of the Prymnesiophyceae, Chrysochromulina parkeae
Green & Leadbeater, 1972 fell within the B. bigelowii
clade, and it was hypothesized that C. parkeae could be thealternate life cycle stage of B. bigelowii sensu stricto. It is
mentioned here because C. parkeae is frequently observedin samples from, e.g. the Isefjord inner broad.
3.5.2 Ceratolithaceae
Ceratolithus cristatus HET coccolithomorpha type
(Lecal-Schlauder, 1950) Young et al., 2003
Pl. 1 Figs 5–6
Only observed once in samples from inner Danish waters(Hals, N. Kattegat, 22 October 1975, surface: 10.4°C, salinity 25.0). The planoliths (Pl. 1, figs 5–6) are morphologicallysimilar to material illustrated in, e.g. Young et al. (2003),
however, markedly smaller in cross section (1.3–1.4µm).
Concluding remarks
The current status with respect to the overall occurrence of
coccolithophore species in the Baltic Sea is summarized inTable 2. This is basically an update of an earlier table provided by Meier et al. (2014). The total number of forms
recorded is in excess of 30. The vast majority of these are
known only from the western Baltic and Kattegat area, andonly a few of these (e.g. Emiliania huxleyi) are of any importance in terms of algal biomass and productivity within
the area. The number of species repeatedly recorded fromthe innermost parts of the Baltic Sea (the Gulf of Finland, theBothnian Sea and the Bothnian Bay) is limited to c. 5 taxa.
In the context of the winter time CaCO3 undersaturation scenario outlined by Tyrell et al. (2008), it is interesting to speculate about why the only coccolithophore species that are
persistently present in the most brackish, open water parts of107

the Baltic Sea are lightly calcified genera and, e.g. represent
species that are otherwise typically associated with polar waters. Is calcification less critical in these apoplastidic forms,
or do they possess, e.g. high amounts of organics (polysaccharides) to protect their coccoliths from dissolution? It is
obvious that further studies, including attempts to culture
these organisms and also an application of a range of cell biological and molecular tools, will be needed to address theseissues beyond mere speculation.
The material examined, in preparation for this compilation of findings of coccolithophore species from the Baltic
Sea, encompasses two unique sampling events that deserve
further comments. The ‘coccolithophore bloom’ encountered in August 1985 when sampling the Isefjord outer broad
remains unique in terms of diversity and abundance of cells.
The community comprised Acanthoica quattrospina HET
and HOL, Balaniger virgulosa HOL, Corisphaera strigilis,
Emiliania huxleyi, Ericiolus spiculiger, and Syracosphaera
borealis. Is the presence of a similar assemblage of coccolithophore species a recurrent phenomenon in the southern
Kattegat and its associated fjords and belts? - or was the August 1985 scenario genuinely unique? There is unfortunately
not material available to answer this question. In as much as
the standard Danish monitoring of phytoplankton relies on
samples fixed in standard acid Lugol’s iodine, the possible
presence of a coccolithophore community has never been
an issue. It is unfortunately also a fact that the number ofsamples available for a TEM/SEM community analysis ofphytoplankton from this area is by far too fragmented to substantially contribute to an overall picture of coccolithophore
occurrence patterns over a time span of decades.
The deep water Øresund sample from September 2014
was unique both in terms of species diversity and abundance of coccolithophores (Acanthoica quattrospina HET,
Algirosphaera robusta, Emiliania huxleyi, Syracosphaera
nodosa), but also in terms of their co-occurrence with anumber of loricate choanoflagellate species that had nothitherto been observed in Danish waters (Thomsen, unpublished results). Some of these were known only fromfaraway places such as the Indian Ocean and thus represent a genuine warm water microbial community. It appears most likely that the September 2014 community
represents a recent major inflow of Atlantic waters into the
Kattegat area. In as much as the reference for the occurrence pattern of loricate choanoflagellates in inner Danish
waters also dates back a couple of decades (Thomsen, unpublished results), it is important to emphasize that what
appears (Sep. 2014) to be a completely new community of
microorganisms in a Kattegat/Belt Sea context might also,
in fact, represent a new ‘normal’. It is a possibility thatclimate change, which impacts, e.g. sea water temperatures and hydrographical flows, may be a decisive factor
in shaping microbial communities in local waters that may
already now appear much different from the old ‘normal’.
The overarching conclusion to be drawn from the observations above is that there is a critical need for new

108ThomsenSpecies

Distribution

SourceAcanthoica quattrospina HET

Kattegat and Belt Sea, Kieler Förde, Isefjord, Øresund

Pankow 1990, Hällfors 2004, Meier et al. 2014, this study

Acanthoica quattrospina HOL

Kieler Förde, Isefjord

Meier et al. 2014, this study

Algirosphaera robusta

Kieler Förde, Øresund

Meier et al. 2014, this study

Alisphaera extenta

Kieler Förde

Meier et al. 2014

Balaniger virgulosa HET

Kattegat and Belt Sea, Gulf of Finland, Bothnian Sea, Bothnian Bay

Thomsen 1979, Thomsen et al. 1988, Ikävalko & Thomsen 1997, this study

Balaniger virgulosa HOL

Kattegat and Belt Sea, Gulf of Finland, Bothnian Sea, Bothnian
Bay

Thomsen & Oates 1978, Thomsen 1979, Thomsen et al. 1988, Ikävalko &Thomsen 1997, this study

Braarudosphaera bigelowii

Kattegat and Belt Sea

Hällfors 2004, this study

Calciopappus caudatus

Kattegat and Belt Sea

Pankow 1990, Hällfors 2004, this study

Calyptrosphaera sp.

Kattegat and Belt Sea

Hällfors 2004 (= Acant. quattrospina HOL?)

Ceratolithus cristatus

KattegatThis study

Coccolithus pelagicus

Arkona Basin, S. Baltic proper, Centr. Baltic proper, N. Baltic
proper, Gulf of Finland, Bothnian Sea, Bothnian Bay

Hällfors 2004

Corisphaera strigilis

Isefjord

This study

Coronosphaera mediterranea

Kattegat and Belt Sea

Hällfors 2004

Discosphaera tubifera

Gdansk Bay (once), ? S. Baltic proper

Pankow 1990, Hällfors 2004

Emiliania huxleyi

Kattegat and Belt Sea, Kiel Bight, Kieler Förde, Isefjord

Pankow 1990, Lohmann 1908, Hällfors 2004, Wasmund et al. 2008,
Meier et al. 2014, this study

Ericiolus spiculiger

IsefjordThomsen et al. 1995

Hymenomonas roseola

Gulf of Finland

Pankow 1990

Hymenomonas sp.

Kieler Förde

Meier et al. 2014

Pappomonas borealis HET

Kattegat and Belt Sea

Thomsen et al. 1988

Pappomonas borealis HOL

Kattegat and Belt Sea

Thomsen et al. 1988

Pappomonas flabellifera HET

Kattegat and Belt Sea

Thomsen et al. 1988

Papposphaera lepida

Kattegat and Belt Sea

Thomsen et al. 1988

Papposphaera arctica HOL

Kategat and Belt Sea, N. Baltic properThomsen et al. 1988, this study

Papposphaera arctica HOL cfr.

Bothnian Sea

This study

Papposphaera iugifera

Bothnian Sea

Thomsen & Østergaard 2016

Pleurochrysis carterae

Kattegat and Belt Sea, Arkona Basin,

Pankow 1990, Hällfors 2004

Pleurochrysis roscoffensis cfr.

Gulf of FinlandThis study

Pseudowigwamma scenozonion

Kattegat and Belt Sea

Thomsen et al. 1988

Syracosphaera borealis

Kieler Förde, Isefjord

Meier et al. 2014, this study

Syracosphaera corolla

Isefjord

This study

Syracosphaera nodosaØresundThis study

Trigonaspis minutissima

Kattegat and Belt Sea

Thomsen et al. 1988

Wigwamma arctica

Kattegat and Belt Sea

Thomsen et al. 1988

Table 2: Summary of species recorded from the Baltic Sea (based on Meier et al., 2014)baseline studies on the marine microbial/nannoflagellate
diversity from Danish waters, i.e. the confluence region
between the Baltic Sea and the North Sea, as an aid in our
understanding of the impacts of climate change on local
ecosystems, and hence also as a general instrument to better be able to evaluate both historical and recent findings.
Acknowledgements

Thanks are due to Guy Hällfors, Åke Niemi and Johanne Ikävalko
for organizing my involvement in sampling campaigns along the
Finnish coastline. Lluisa Cross and Elisa Malinverno are acknowledged for providing valuable advice with respect to theidentification of some coccolithophore species. Thanks to Mikal
Heldal for allowing us to use his SEM image of B. ­virgulosa
HET. Thanks are finally due to two anonymous reviewers forproviding a wealth of ideas to improve the paper.

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ThomsenPlate 1

Coccolithophores from inner Danish waters (Figs 1–11) and Finland (Figs 12–13). SEM: Figs 1, 7, 11; TEM: Figs 2–6, 12–13; LM (Light Microscope): Figs 8–10 (phase contrast and nomarski interference contrast); reverse printing: Figs 4, 6, 13.
Fig. 2. Complete coccosphere. Fig. 3. Cell with flagella and haptonema. Fig. 4. Uncalcified under layer scales. Fig. 8. Phase contrast. Fig 9. Live cell.
Fig. 10. Detached liths (Nomarski interference contrast).

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Plate 2 – Acanthoica quattrospina

HOL (Figs 1–6) and HET (Figs 7–8) from Danish waters; TEM: Figs 1, 3–6, 8; SEM: Figs 2, 7; reverse printing: Figs 4–5.
Fig. 1. Complete cell. Fig. 2. Calyptroliths. Fig. 3. Calyptroliths in TEM. Notice the curled up haptonema. Fig. 4. Unmineralized large and small
under layer scales. Fig. 5. Unmineralized scale carrying columnar or ‘worm-like’ material. Fig. 6. Scatter of calyptroliths and under layer scales. The
small scales tend to occur along the periphery of the larger unmineralized scales. Fig. 7. Complete coccosphere. Fig. 8. Detail of body coccoliths.

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ThomsenPlate 3

TEM: Figs 1–5; reverse printing: Figs 4–6.
Fig. 1. Complete cell. Fig. 2. Coccosphere; notice the curled up haptonema next to a single flagellum. Figs 3–4. Coccoliths. Fig. 5. Scatter of zygoliths
and unmineralized scales. There is a gradual decalcification from right to left which makes it probable that the large unmineralized scales (arrows) arein fact the organic scale supporting the zygoliths. These are in turn ‘encircled’ by numerous and small unmineralized scales.

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Plate 4 – Corisphaera strigilis cfr.

TEM images from the Isefjord; reverse printing: Figs 2–5.
Fig. 1. Complete cell. Fig. 2. Detail of coccoliths from a vacated coccosphere. Most coccoliths appear somewhat disturbed. The most prominent unifying feature is a peripheral band of densely packed crystallites. Unmineralized under layer scales are present (lower right). Fig. 3. Weakly calcified
coccoliths overlaying small unmineralized scales. Fig. 4. Complete cell. Notice the cluster of unmineralized scales that are visible at the antapicalpole. A couple of fairly well defined zygoliths are pointed out. Fig. 5. Well-developed zygolith.

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ThomsenPlate 5 – Balaniger virgulosa HOL

TEM images from Danish inner waters.
Fig. 1. Reversed print showing the typical appearance of the B. virgulosa pyramids. Some pyramids (arrows) have a distinct angular distal end whichindicates that a minute crystallite might be positioned here. Fig. 2. High magnification of a single pyramid. Fig. 3. Detail of complete cell with flagellaand haptonema showing the characteristic grouping of pyramids in clusters that match individual supporting organic scales. Fig. 4. Weakly mineralized specimen with flagella and haptonema. Fig. 5. Detail (from Fig. 4) of organic scales with traces of pyramids (arrow).

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Plate 6 – Balaniger virgulosa HET and HOL

From the Baltic proper (Figs 1–2), Norway (Fig. 3) and Denmark (Figs 4–5). TEM: Figs 1–2, 4–5; SEM: Fig. 3; reverse printing: Figs 2, 4.
Figs 1–2. From the Baltic proper; Fig. 3. Norway; Figs 4–5. Denmark. Image type: TEM - Figs 1–2, 4–5; SEM - Fig. 3; Reversed printing - Figs 2, 4.
Fig. 1. Complete cells of P. virgulosa HET (top) and P. virgulosa HOL (bottom). Fig. 2. Scatter of heterococcoliths and holococcoliths (pyramids) indicatingthat this might be a broken combination coccosphere. Notice that the individual pyramid is sharp pointed and fairly different from those shown in e.g. Pl. 5,
fig. 1. Fig. 3. Complete coccosphere with a distinct cluster of calicate flagellar pole coccoliths. Fig. 4. Detail of body coccolith. Fig. 5. Complete cell.

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ThomsenPlate 7 – Balaniger virgulosa HET

TEM images from the Baltic.
Fig. 1. Complete coccosphere. The circumflagellar coccoliths have long calicate spines (top left) while other coccoliths have calicate spines in which
the stem is significantly shortened. Figs 2–3. Details of coccospheres in which aberrant types of calyces mix in between normal ones. Fig. 4. Specimen
from the most brackish part of the Baltic Sea. Body coccoliths are without a central spine. The arrow points to an antapical pole coccolith that carries
a reduced central appendage Fig. 5. A second coccosphere which shares with Fig. 1 the presence in all coccoliths of a central calicate spine. Fig. 6.
Reverse printing. Detail of body coccolith calcification.

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Plate 8TEM images from inner Danish waters; reverse printing: Figs 3, 6.
Fig. 1. Complete cell. Fig 2. Complete coccosphere in which some of the body coccoliths have broken loose. Fig. 3. Detached body coccoliths showing the triangular crystallite groups united to form regular patterns. Fig. 4. Complete cell. Fig. 5. Detail of flagellar pole coccoliths. Fig. 6. Detail from
Fig. 5 showing the triangular crystallite groups. Fig. 7. Flagellar pole coccoliths. Fig. 8. Complete cell. Fig. 9. Body coccoliths. Fig. 10. Complete cell.

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ThomsenPlate 9Papposphaera arctica HOL and combination coccosphere from inner Danish waters (Figs 1–5/TEM images). Papposphaera arctica HOL cfr. fromthe Bothnian Sea (Figs 6–8/TEM images); reverse printing: Figs 5, 8.
Fig. 1. Complete cell with flagella and haptonema. The unilateral distal proliferation of the flagellar pole coccoliths is only little pronounced. Fig. 2. Flagellar
pole coccoliths (detail from Fig. 3); notice the unilateral distal hypertrophy. Fig. 3. Complete coccosphere. Figs 4–5. Combination coccosphere. The area that
displays both hetero- and holococcoliths is marked in Fig. 4, and enlarged in Fig. 5. Fig. 6–7. Complete cells. Fig. 8. Detail from Fig. 6 showing a coccolithfrom the antapical pole. The hexagonal groups of crystallites are evident.

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Plate 10

Images from inner Danish waters; reverse printing: Figs 2, 4.
Fig. 1. Pappoliths. Fig. 2. Detail of the collar at the distal end of the stem. Fig. 3. Complete cell. Fig. 4. Detail from Fig. 3 showing the rim calcificationin the species. Fig. 5. Complete cell.